Ocean Acidification

An international team of researchers, led by a geologist at the University of Greifswald has investigated unusually well-preserved fossils of reef corals from the subtropical Central Paratethys Sea to analyse their ability to withstand ocean warming and acidification during the Middle Miocene approximately 16 to 13 million years ago. This period was characterised by raised levels of carbon dioxide in the atmosphere and a globally warmer climate – similar to scenarios that are expected in the future of our planet.

The researchers from the Universities of Greifswald, Leipzig, and Mainz, as well as the National Autonomous University of Mexico, used seasonal geochemical and growth records to reconstruct how the fossil corals responded to environmental changes during the Middle Miocene. They presented their results in a scientific article recently published in Communications Earth & Environment. The corals could actively regulate the pH value and the saturation level of their internal calcifying fluid. Thus they had a mechanism that helped them to withstand adverse environmental conditions. However, this physiological adaptation did not enable them to compensate the unfavourable conditions in full: “The corals had an extremely low growth rate and their skeletons were only weakly calcified. We assume that this had a considerable impact on the development of reef structures,” explains Dr. Markus Reuter, Research Associate in the field of palaeontology at the University of Greifswald and lead author of the study.

As global climate change intensifies, ocean acidification is becoming a ‘relentless killer’ threatening coral reef ecosystems. Recently, a research paper published in the international authoritative journal Research has revealed diverse survival strategies of reef-building corals in response to ocean acidification, providing a new perspective for understanding and protecting this fragile marine ecosystem.

Since the Industrial Revolution of the Anthropocene, human activities have led to a continuous decline in global ocean pH levels. According to predictions, by the end of this century, the global average seawater pH may drop from 8.0–8.2 to 7.6–7.8, posing a lethal survival crisis for marine organisms that rely on calcium carbonate skeleton systems, especially reef-building corals. While existing research has confirmed that ocean acidification reduces skeletal density and growth rates of reef-building corals, there have been no reports on the survival strategies of different coral species in response to acidified marine environments, particularly the precise dynamic changes of their internal skeleton and canal structures.

To explore these issues in depth, the research team simulated an acidified marine environment with pH values of 7.6–7.8 in the laboratory. They selected four species of reef-building corals widely distributed in the Indo-Pacific region: Acropora muricata, Pocillopora damicornis, Montipora capricornis, and Montipora foliosa. Using high-resolution micro-computed tomography (micro-CT), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and transcriptome sequencing (RNA-seq) detection technologies, the researchers conducted multidimensional integrated analysis of the skeletal erosion process, elemental dynamic changes, and gene expression states of these reef-building corals under acidified marine environments.

Species-Specific Survival Strategies

The results suggest that different reef-building coral species have diverse growing strategies in lower pH conditions. A. muricata demonstrated its unique ‘cavity-like’ acid erosion strategy, while the other three species developed degradation characteristics similar to ‘osteoporosis’ in human aging processes, exhibiting disordered skeletal structures, insufficient synthesis of adhesion proteins, and low bone mass, correspondingly.

In a surprising twist for marine science, researchers have discovered that copepods—tiny but crucial creatures at the base of the ocean food chain—use not one but two molecular toolkits to survive in a warming, acidifying ocean.

The discovery reveals a two-pronged strategy: one genetic, the other epigenetic, that helps these animals rapidly adjust and evolve across generations.

The findings, published July 15 in Proceedings of the National Academy of Sciences, offer a rare dose of optimism in climate research. Led by Melissa Pespeni at the University of Vermont, the study tracked 25 generations of marine copepods under simulated future ocean conditions. The result? Clear evidence that these organisms are not just adapting genetically over time, but also deploying rapid, reversible changes through epigenetic modifications—chemical tags on DNA that influence which genes get expressed.

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The team raised populations of Acartia tonsa—a globally abundant copepod species—in lab conditions mimicking ocean warming, acidification, and their combination. Over one year and 25 generations, researchers measured everything from egg production to genome-level changes. Using cutting-edge sequencing, they tracked:

• Genetic adaptation (DNA sequence changes)
• Epigenetic variation (DNA methylation)
• Gene expression patterns (which genes were turned on or off)

What they found was startling: genetic and epigenetic changes occurred in different regions of the genome and seemed to operate independently. Yet both mechanisms contributed to the copepods’ ability to tolerate stressful environments.

The resistance or vulnerability of marine organisms’ shells to ocean acidification does not depend only on the type of mineral they are made of, as previously thought, but also on factors such as their microstructure and organic content. This is the main conclusion of a study by the ICTA-UAB, which calls for a reassessment of current scientific models.

Ocean acidification — driven by increasing atmospheric CO₂ — has become a critical threat to marine life, particularly for organisms that build calcium carbonate shells. For years, it has been widely assumed that organisms with aragonitic shells (a more soluble form of Calcium carbonate CaCO₃) are more vulnerable than those with shells made of calcite (a less soluble form). However, this assumption was based on the behaviour of synthetic monocrystals produced in inorganic precipitation experiments. Calcium carbonate shells, by contrast, are highly complex structures containing organic material as well as minerals. A new study reveals a far more complex reality challenging oversimplified assumptions based synthetic monocrystals

The research shows that the vulnerability of these organisms cannot be predicted based solely on the mineralogy of their shells. Instead, other factors — such as the shell’s microstructure and organic content — are also critical for understanding how these structures respond to undersaturated and corrosive conditions.

“We have a generalized idea about the impact of ocean acidification on marine shells, but it’s not enough to know whether they’re made of aragonite or calcite. It also matters how they’re built,” explains Gerald Langer, ICTA-UAB researcher and lead author of the study. The way organisms build their shells — including internal structure and organic matter — varies between species and can significantly influence their resistance to more acidic seawater.

The experimental evidence analyzed by the team includes cases where structures made of the same mineral exhibit very different dissolution behaviours, depending on their internal design or the presence of organic coatings. A paradigmatic example is that of coccolithophores, where the same species shows variable shell resistance depending on its life cycle stage, even though all phases use the same type of calcium carbonate.

This finding has important implications for conservation policies and for scientific models predicting the impacts of climate change on marine biodiversity. Many of these models use mineralogy as a direct proxy for vulnerability — a practice that, according to this new research, needs to be fundamentally re-evaluated.

“This study challenges one of the foundations of relevant scientific assessments used by international bodies, by showing that mineralogy alone does not predict the resilience of calcifying species in acidifying oceans,” says Patrizia Ziveri, ICTA-UAB research professor and co-author of the study. ““As oceans continue acidifying, improving our understanding of which species are most at risk is essential for designing effective protection strategies,” Ziveri adds.

“The signs of the ocean in distress are all around us”, said Peter Thomson, Special Envoy of the Secretary-General of the United Nations for the Ocean, at the conference in Nice, France last week. “The time of debating with the denialists is over”. This statement of intent backed a slew of agreements that aim to remedy the damage already done to our oceans and prevent further harm. Overfishing, deep-sea mining, pollution with a focus on plastic, with acidification and other threats associated with climate change were on the agenda of global problems to eliminate. We all knew these problems were bad and getting worse, but a new report reveals that we continue to race past critical tipping points without even realizing it.

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When carbon dioxide is released into the atmosphere about 25% of it will end up in the oceans where it dissolves, and makes the water more acidic. This change in pH is as drastic a change in the environment as temperature or any other abiotic factor. This new aquatic chemistry isn’t something to which organisms can adapt, and they’re correspondingly suffering. Comparing certain species of shellfish with their pre-Industrial Revolution conspecifics (members of the same species) collected in 1875 by the crew of the HMS Challenger, we see that they’re up to 76% thinner than their ancestors. This was observed with a 40% increase in acidity. By 2100 the oceans will be 150% more acidic than at present.

This “Evil Twin” of Climate Change has been underestimated. We’re realizing that we’ve crossed a significant tipping point five years ago. Ocean Acidification is so dramatic that shellfish larvae can’t form their shells. We now understand that the acidity interferes with the creation of calcium carbonate that’s need to form the shells of these organisms. Oyster farmers from the Pacific Northwest have observed this since at the early 2000s. Coral, Crabs, and Krill are some of the organisms that have been specifically studied and seen to be struggling. It would be sad in and of itself if we couldn’t enjoy delicious crabs and mussels anymore, but this is worse when we consider that it signals a pending, global ecological catastrophe.

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Why is this happening?

We’ve been abundantly releasing carbon since the Industrial Revolution. Even with man-made global warming being hypothesized in the late 1800s, it’s taken a long time for us to feel its effects. Part of this is because significant amounts of carbon dioxide get absorbed into the oceans, from atmosphere to ocean surface, removing its warming potential. The oceans are so vast that as of 2010 they were storing about 16 times as much of carbon as makes up all living plants and animals on earth. This quantity is 60 times the amount that was in the atmosphere before the Industrial Revolution.

The oceans will have a saturation point at which they can’t absorb more carbon. That can be a concern eventually but don’t worry, that won’t happen until about a pH of 7.5. The majority of marine life will be dead by then. How serious of a problem is this? How likely are we to reach levels where even our biggest carbon reserve can’t take it anymore? If not likely any time soon, it’s a real enough of a question that scientists are proposing a new indicator to quantify ocean acidification.

Gamma Subscript CO2

In this paper published in May of this year, scientists propose a new variable (γ_CO2) to represent the absorption potential of the oceans. This is needed because as stated, we have pumped so much carbon into the atmosphere, which in turn absorbs into the oceans, that we need new math to do future calculations. Without modifying our current trajectory, we could reach a pH of 7.8 by 2100, which would be comparable to 14-17 million years ago when our planet was in the midst of an extinction event.

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At the dawn of the millennium, a group of eminent scientists began compiling a list of the threats they felt were most likely to impact the world’s rocky shorelines over the coming quarter of a century.

Published in 2002, it included forecasts that – among other things – pollution from oil spills would decrease, the number of invasive species across the world would rise, genetically-modified organisms would have harmful effects on the ocean, and the impacts of global climate change would be felt more intensely.

Now, 25 years on, the same academics – along with a larger and more wide-ranging team of international experts – have revisited their forecasts and discovered that many of them were correct, either in whole or in part, while others haven’t had the impacts that were envisaged at the time.

They have also charted some of the other threats to have emerged and grown in significance since their original work, with notable examples including global plastic pollution, ocean acidification, extreme storms and weather, and light and noise pollution.

In doing so, they have also highlighted that while there are key issues they believe are likely to threaten the world’s coastlines between now and 2050, others may also emerge that require varying levels of local and global action to try and tackle them.

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What the scientists missed

• The impacts of coastal mining;
• Ocean acidification and its potential impact on marine species;
• The effects of artificial light pollution;
• The effects of noise pollution;
• Extreme flood and drought events;
• The scale and effects of plastic pollution;
• The impacts of pharmaceutical contamination;
• The combined effects of various environmental threats and chemical compounds.

The full study – Hawkins et al: Hindsight informs foresight: revisiting millennial forecasts of impacts and status of rocky shores in 2025 – is published in Marine Pollution Bulletin, DOI: 10.1016/j.marpolbul.2025.118214.

The world’s oceans are in worse health than realised, scientists have said today, as they warn that a key measurement shows we are “running out of time” to protect marine ecosystems.

Ocean acidification, often called the “evil twin” of the climate crisis, is caused when carbon dioxide is rapidly absorbed by the ocean, where it reacts with water molecules leading to a fall in the pH level of the seawater. It damages coral reefs and other ocean habitats and, in extreme cases, can dissolve the shells of marine creatures.

Until now, ocean acidification had not been deemed to have crossed its “planetary boundary”. The planetary boundaries are the natural limits of key global systems – such as climate, water and wildlife diversity – beyond which their ability to maintain a healthy planet is in danger of failing. Six of the nine had been crossed already, scientists said last year.

However, a new study by the UK’s Plymouth Marine Laboratory (PML), the Washington-based National Oceanic and Atmospheric Administration and Oregon State University’s Co-operative Institute for Marine Resources Studies found that ocean acidification’s “boundary” was also reached about five years ago.

“Ocean acidification isn’t just an environmental crisis – it’s a ticking timebomb for marine ecosystems and coastal economies,” said PML’s Prof Steve Widdicombe, who is also co-chair of the Global Ocean Acidification Observing Network.

The study drew on new and historical physical and chemical measurements from ice cores, combined with advanced computer models and studies of marine life, which gave the scientists an overall assessment of the past 150 years.

It found that by 2020 the average ocean condition worldwide was already very close to – and in some regions beyond – the planetary boundary for ocean acidification. This is defined as when the concentration of calcium carbonate in seawater is more than 20% below preindustrial levels.

Researchers have found that ocean acidification entered a “danger zone” in 2020, suggesting increased carbon dioxide levels have caused Earth to breach another planetary boundary.

The new study suggests our planet’s oceans are becoming too acidic to remain healthy. (Image credit: Philip Thurston via Getty Images).

Earth’s oceans are in worse condition than scientists thought, with acidity levels so high that our seas may have entered a “danger zone” five years ago, according to a new study.

Humans are inadvertently making the oceans more acidic by releasing carbon dioxide (CO₂) through industrial activities such as the burning of fossil fuels. This ocean acidification damages marine ecosystems and threatens human coastal communities that depend on healthy waters for their livelihoods.

Previous research suggested that Earth’s oceans were approaching a planetary boundary, or “danger zone,” for ocean acidification. Now, in a new study published Monday (June 9) in the journal Global Change Biology, researchers have found that the acidification is even more advanced than previously thought and that our oceans may have entered the danger zone in 2020.

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The researchers concluded that by 2020, the average condition of our global oceans was in an uncertainty range of the ocean acidification boundary, so the safety limit may have already been breached. Conditions also appear to be worsening faster in deeper waters than at the surface, according to the study.

“Ocean acidification isn’t just an environmental crisis — it’s a ticking time bomb for marine ecosystems and coastal economies,” Steve Widdicombe, director of science and deputy chief executive at Plymouth Marine Laboratory, a marine research organization involved in the new study, said in a statement. “As our seas increase in acidity, we’re witnessing the loss of critical habitats that countless marine species depend on and this, in turn, has major societal and economic implications.”

In 2009, researchers proposed nine planetary boundaries that we must avoid breaching to keep Earth healthy. These boundaries set limits for large-scale processes that affect the stability and resilience of our planet. For example, there are boundaries for dangerous levels of climate change, chemical pollution and ocean acidification, among others.

A 2023 study found that we had crossed six of the nine boundaries. The authors of that study didn’t think the ocean acidification boundary had been breached at the time, but they noted it was at the margin of its boundary and worsening.

Katherine Richardson, a professor at the Globe Institute at the University of Copenhagen in Denmark who led the 2023 study and was not involved in the new study, told Live Science that she was “not at all surprised” by the new findings.

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What causes ocean acidification?

Ocean acidification is mostly caused by the ocean absorbing CO₂. The ocean takes up around 30% of CO₂ in the atmosphere, so as human activities pump out CO₂, they are forcing more of it into the oceans. CO₂ dissolves in the ocean, creating carbonic acid and releasing hydrogen ions. Acidity levels are based on the number of hydrogen ions dissolved in water, so as the ocean absorbs more CO₂, it becomes more acidic.

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LiveScience, 12 june 2025. Press release.

Princeton University and Xiamen University researchers report that in tropical and subtropical oligotrophic waters, ocean acidification reduces primary production, the process of photosynthesis in phytoplankton, where they take in carbon dioxide (CO₂), sunlight, and nutrients to produce organic matter (food and energy).

A six-year investigation found that eukaryotic phytoplankton decline under high CO₂ conditions, while cyanobacteria remain unaffected. Nutrient availability, particularly nitrogen, influenced this response.

Results indicate that ocean acidification could reduce primary production in oligotrophic tropical and subtropical oceans by approximately 10%, with global implications. When extrapolated to all affected low-chlorophyll ocean regions, this translates to an estimated 5 billion metric tons loss in global oceanic primary production, which is about 10% of the total carbon fixed by the ocean each year.

The research is published in the journal Proceedings of the National Academy of Sciences.